Number portability is the ability of a customer owning a particular fixed or mobile telephone number to reassign the telephone number to another carrier (“Service Provider Portability”), move it to another location (“Geographic Portability”), or change the type of service (“Service Portability”). The customer is therefore able to change these attributes of their service without having to change the telephone number which they are using.
A variable numbering plan is a type of numbering scheme used in telecommunications to assign telephone numbers to subscribers, where the length of each telephone number is not fixed.
Alternatively, closed numbering plans impose a specific number of digits for each telephone number. Different numbering plans are operated in different countries and territories.
In both variable and closed numbering plans, overlap dialling can be implemented. Overlap dialling is when a telephone number can be specified partially to complete the first stage of a particular routing path. In other words, the number may be formed of a plurality of separate messages, e.g. an Initial Address Message (IAM) and one or more Subsequent Address Messages (SAMs) or e.g. multiple Session Initiation Protocol (SIP) INVITEs.
For a variable numbering plan, overlap dialling is an important feature as the length of the complete telephone number is unknown. The end of the dialling can be signalled by an inter-digit timer, or some other method, for example if it is known that a particular series has a particular length. In overlap dialling, a first part of the telephone number is received from the calling party with only a small delay between the reception of the digits and a second part may be received in overlap mode where an inter-digit timer watches a maximum time between the digits until the complete number has been received.
However, if the use of an inter-digit timer for signalling the end of dialling by a time out, in order to perform number porting is not accepted, it is necessary to determine an alternative way to identify whether or not a number is ported, as it is not clear when the end of a dialled number occurs. In particular when the length of the number to be ported is not known, waiting for a presumed last digit before the number porting is performed is regarded a problem.